Coextrusion of a two-layer polystyrene structure having a foam layer and a solid unfoamed layer has been a common commercial operation for many years. These processes employ an annular die where the extrudate is drawn as a sleeve or tube out over a mandrel on which the material is cooled, sized in width and formed into a sheet. However, in all such cases, the unfoamed layer has to be kept relatively thin (0.5-0.75 mils) and the foam layer kept relatively thick (general minimum around 4.0 mils). In other words, the foam layer has been the predominate portion of the total thickness and the unfoamed layer the minor portion of the total thickness. This type of structure lacks in durability due to the predominate portion being foam, and is sensitive to tearing and chipping especially when conveying, accumulating and boxing filled labeled containers for commercial use. Accordingly, it has been used as labels in environments where the label does not require durability.
Attempts have been made to make the unfoamed layer thicker while maintaining the foamed layer at the same thickness but this has resulted in an unsuitable structure due to brittleness and roughness.
Attempts have been made to add resin additives to the unfoamed layer in order to toughen the structure but this has proved of little benefit because the foamed layer, being the thicker, results in the structure lacking the durability as described above.
Attempts have been made to use thin unfoamed polystyrene films of 5 mils or less for label material. Such material has a density of 64-65 PCF and is unsuitable for use in the recycling of the PET plastic bottles. In addition, rolls of thin film material less than 5 mils have gauge bands and are uneven in hardness across the width of the roll which creates problems in subsequent processing such as printing and cutting into labels.
Attempts have been made to use a single thin unfoamed plastic film such as polypropylene or polyethylene. However, such a label material has disadvantages among which is the tendency of the thin film, having a thickness of less than 5 mils, to have gauge bands. When such a material is extruded and wound in a roll, the rolls tend to deviate from a cylinder and are uneven in hardness across the width of the roll. In addition, the thin film lacks stiffness and cannot be readily handled in the subsequent processing such as printing and cutting into labels.
Thin films (3.0 mils or less) made from polystyrene with any functional utility for label stock with a density less than 64 PCF (the natural density of polystyrene) have previously not been possible to make; again due to the brittle nature of blown expanded polystyrene.
Among the objectives of the present invention are to provide a label material which can be printed and decorated which has greater durability and is less sensitive to tearing and chipping during handling. Among the further objectives are to provide a plastic substrate which can be used as a label material or as a substrate by laminating another unfoamed plastic layer thereon by adhesive or by extrusion coating; wherein the presence of gauge bands is minimized; wherein the label material can be wound in rolls that can be readily handled without the adverse effects of gauge bands due to the unfoamed plastic layer or due to ink printing thereon; wherein the labels have a thin layer of foam material which provides cells that define an irregular surface which is receptive to adhesive for adhering the label to a container such as a glass bottle, plastic bottle or can; wherein a minimal amount of adhesive is required to adhere the label; wherein if the label is not applied correctly, the label can be readily removed from the container and a minimal portion of the label is left on the container such that the container can be re-labeled; wherein when the container with the label is recycled, the density of the label is below that of water such that the label can be readily reclaimed separate from the container; wherein the label has greater stiffness than a thin unfoamed layer; and wherein when the label material is to be used as a pressure sensitive layer and is applied to a carrier web for handling, the label and carrier web provide sufficient insulation to withstand the heat of laser printing as is sometimes used in the field as, for example, in labeling pressure sensitive labels used on store shelves.
In accordance with the invention, an extruded thermoplastic thin sheet material is produced that is suitable for use as labeling stock to be applied to glass or plastic containers. Requirements of such labeling stock are surface smoothness to provide a high quality printing surface, toughness to resist chipping and tearing in commercial high speed conveying and packaging of filled bottles, and waterproofness for durability in consumer use. An additional preferred attribute of the label stock is that of a thin overall thickness or caliper (1.5-3.0 mils) whereby the number of labels on a roll can be greater than with a thicker caliper so that in use the number of roll changes necessary in a high speed labeling operation are reduced.
In accordance with one aspect of the present invention, a coextruded label material is provided in a substrate which comprises a styrenic unfoamed layer and a styrenic foam layer wherein the styrenic unfoamed layer is the predominant portion as contrasted to the styrenic foam layer. In accordance with another aspect of the invention, an additional clear or translucent unfoamed plastic layer is adhered to the unfoamed layer with printing interposed between the unfoamed layers. Printing is provided on the unfoamed layer of the coextruded material or other side of the additional clear or transparent unfoamed layer if applied by lamination.
In accordance with another aspect of the invention, labels from a coextruded substrate are provided with a pressure sensitive adhesive on the thin foam layer and the labels are, in turn, adhered by the pressure sensitive adhesive onto a carrier for handling the label.